Fiber optic waveguides are a viable, relatively broad bandwidth alternatives to the coaxial cable transmission lines that are now in widespread use in communication networks. As is known, there are single mode and multi-mode, single fiber and multi-fiber optical waveguides. Single mode fibers have a superior bandwidth/length characteristic, but are difficult to splice and repair because of their relatively small core diameters (typically 2-20 .mu.m). They may be used to advantage in relatively long haul, high speed communication networks-viz., networks having a length in excess of several km and a data rate higher than about 200 Mbits/sec. Otherwise, however, multi-mode fibers are the optical waveguides of choice because of their greater core diameters (typically 40-400 .mu.m)
Multi-mode fiber optic bundles (i.e., multi-fiber waveguides) are particularly well suited for relatively short length, low speed communication neworks. The inherently provide redundancy because of their multi-fiber construction and they are resonably easy to efficiently couple to available light sources, such as light emitting diodes (LEDs) and diode lasers, because of their relatively large cross sectional areas. However, they suffer from the disadvantages of (1) having a cost which increases as a function of the number of fibers in the bundle, (2) requiring substantial clearances because of their relatively large cross sectional areas, and (3) often having a relatively large packing injection loss due to the claddings for the several fibers and the inactive areas therebetween. The packing injection losses usually can be reduced to an extent at the expense of increased manufacturing costs, but such costs are frequently difficult to justify. of coupling sufficient light into multi-mode, single strand, fiber optic waveguides for transmission over distances of up to several km without repeatering. Thus, such waveguides have become the preferred optical communications medium for use in most medium to high speed local area networks.
Linear network architectures are becoming increasingly popular, at least in part as a result of the trend toward distributed processing. For example, the Xerox Ethernet local area network has an open loop linear architecture so that terminals (e.g., workstations; shared resources, such as printers and the file servers; and other types of processors) may be added to the network simply by tapping into a coaxial cable communications medium. One of the features of the standard Ethernet network is that fail-safe taps are used so that there is through transmission along the network even if there is a local power failure.
A rugged and reliable optical T-coupler is required for coupling local terminals to single strand, single or multi-mode fiber optic waveguides if such waveguides are to serve as the communications media for linear optical communication networks. Indeed, to provide a full optical counterpart to existing coaxial cable networks, such as the Zerox Ethernet network, a fail-safe coupler is needed. That, of course, rules out optical couplers which rely on active repeaters for through transmission.
Others have proposed fail-safe optical T-couplers which are suitable for coupling local terminals to single strand fiber optic waveguides. The twin T-coupler Ueno and Oogi described at the May 1976 CLEOS Conference in San Diego, Calif. in a paper entitled "Data Highway Using Optical Fiber Cable" is an especially relevant example. In particular, that coupler is configured for coupling local terminals to a dual bus, bidirectional network. To that end, it comprises four SELFOC collimating lenses and a beam splitting prism which are assembled so that each bus is connected to the prism by two of the SELFOCs. Furthermore, in accordance with Ueno and Oogi's teachings, each terminal has a pair of lasers or LED light sources and a pair of photodiodes which are aligned on opposite sides of the beam splitting prism of the associated coupler. Unfortunately, such a coupler does not efficiently couple locally injected light into the buses, even if the light sources are imaged onto the beam splitting prism. It may be adequate for relatively short haul communications over single strand, multi-mode waveguides if relatively high powe, well collimated light sources and relatively sensitive photodetectors are used. Nevertheless, a more efficient coupler is clearly needed. The acousto-optic modulator/coupler shown in U.S. Pat. No. 3,920,982, which issued Nov. 18, 1975 on a "Continuous Fiber Optical Transmit and Receive Termminal," is also of some interest. It relies on bulk acoustic energy for modulating and scattering light guided by a continuous, single or multi-fiber optical waveguide. Otherwise, however, that device is of no particular relevance because it does not have the ability to couple locally injected light into the waveguide.